UM10503
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
128 of 1269
NXP Semiconductors
UM10503
Chapter 11: LPC43xx Clock Generation Unit (CGU)
•
mode 1a: Normal operating mode without post-divider and without pre-divider
•
mode 1b: Normal operating mode with post-divider and without pre-divider
•
mode 1c: Normal operating mode without post-divider and with pre-divider
•
mode 1d: Normal operating mode with post-divider and with pre-divider
To get at the output of the PLL (clkout) the best phase-noise and jitter performance, the
highest possible reference clock (clkref) at the PFD has to be used. Therefore mode 1a
and 1b are recommended, when it is possible to make the right output frequency without
pre-divider.
By using the post-divider the clock at the output of the PLL (clkout) the divider ratio is
always even because the divide-by-2 divider after the post-divider.
11.7.4.3.2
Mode 1a: Normal operating mode without post-divider and without pre-divider
In normal operating mode 1a the post-divider and pre-divider are bypassed. The operating
frequencies are:
Fout = Fcco = 2 x M x Fin
(275 MHz
Fcco
550 MHz, 4 kHz
Fin
150 MHz)
The feedback divider ratio is programmable:
•
Feedback-divider M (M, 1 to 2
15
)
11.7.4.3.3
Mode 1b: Normal operating mode with post-divider and without pre-divider
In normal operating mode 1b the pre-divider is bypassed. The operating frequencies are:
Fout = Fcco /(2 x P) = (M / P) x Fin
(275 MHz
Fcco
550 MHz, 4 kHz
Fin
150 MHz)
The divider ratios are programmable:
•
Feedback-divider M (M, 1 to 2
15
)
•
Post-divider P (P, 1 to 32)
11.7.4.3.4
Mode 1c: Normal operating mode without post-divider and with pre-divider
In normal operating mode 1c the post-divider with divide-by-2 divider is bypassed. The
operating frequencies are:
Fout = Fcco = 2 x M x Fin / N
(275 MHz
Fcco
550 MHz, 4 kHz
Fin/N
150 MHz)
The divider ratios are programmable:
•
Pre-divider N (N, 1 to 256)
•
Feedback-divider M (M, 1 to 2
15
)
Table 93.
DIRECTL and DIRECTO bit settings in HP0/1_Mode register
Mode
DIRECTI
DIRECTO
1a
1
1
1b
1
0
1c
0
1
1d
0
0
